Electron spin resonance

Electron spin resonance (ESR) dating is also based on the accumulation of radiation-induced energy in minerals and thus has close links to luminescence dating. Although first attempts to exploit the ESR phenomenon for dating go back to the 1960s (Zeller et al. 1967), as a dating method it did not flourish before the 1980s and is still being developed. The ESR method permits age determination up to few million years, far beyond the range of the luminescence methods, and covers the whole Quaternary period. The most important material for paleoanthropologic ESR application is tooth enamel, but quartz-separates from sediments at prehistoric sites also have a certain potential (Rink 1997). With ESR, one dates either the burial, as in the case of fossil teeth, or the resetting of a previous system as a result of bleaching, as in the case of sedimentary quartz grains or heating of stones.

The ESR phenomenon is caused by paramagnetic centers in the crystal lattice. Radiation-induced trapped electrons, mentioned already in the context of luminescence dating, form such centers and give rise to characteristic ESR signals. The intensity of the ESR signal is a function of the number of trapped electrons and, therefore, of the accumulated energy dose AD that has been absorbed from the ionizing radiation in the course of time. In order to calculate the ESR age, the value of AD, obtained from the ESR measurement, is divided by the dose rate DR, in the same manner as already discussed for luminescence (O Eq. 7).

The quantity of AD is determined by ESR spectrometry, which exploits the fact that trapped electrons are unpaired. Brought into a variable magnetic field and exposed to a given microwave, unpaired electrons show spin resonance at a specific strength of the magnetic field. The condition at which resonance happens is described by the g-value, which is characteristic for the type of the paramagnetic center. The energy necessary for the resonance is absorbed from the microwave so that its intensity reduction is a measure for the concentration of the center. The resulting ESR spectrum shows the specific microwave absorption for various centers with different g-values. Owing to measurement-technical reasons, the ESR spectra of the microwave absorption are not directly recorded; instead their first derivation as a function of the field strength is plotted. For the evaluation of AD, known doses are applied additively to the sample and a growth curve is established. ESR has the advantage over luminescence that the concentration of the probed centers is not disturbed by the measurement procedure, thus permitting one to establish the growth curve on the same aliquot. Most samples show exponential saturation functions. At normal ambient temperatures of sediments, most ESR centers are sufficiently stable for applying ESR dating up to few million years. The dose-rate determination follows the same principles as already mentioned for luminescence. For ESR-dating samples of a few grams are sufficient, provided the material is homogeneous, but usually larger sample sizes are preferable as they allow one to separate suitable materials in the laboratory and to conduct microdosimetric measurements.

ESR dating of fossil tooth enamel plays an important role for Paleolithic sites, mainly because teeth are commonly preserved. Furthermore, the age range covered by ESR dating reaches beyond those of radiocarbon or luminescence dating so that even the Lower Paleolithic becomes accessible. Mammalian dental issue consists essentially of enamel, dentine, and cementum layers. ESR dating is based on the mineral hydroxyapatite, in particular on its carbonate-containing subspecies dahllite, within the enamel. Dentine and cementum are less dense, contain more organic tissue, and take up uranium more easily than dentin. For these reasons, they are not used for ESR analysis but must be taken into account for microdosimetric reasons. The ESR spectrum of tooth enamel has, at g = 2.0018, a suitable signal of good sensitivity and high thermal stability.

The quantity of the dose rate introduces considerable problems into the ESR age evaluation. In teeth, one observes often strongly varying uranium contents on microscopic scale causing steep dose-rate gradients, which is in particular the case for the b-component since the b-radiation range (ca. 1 mm) is similar to the thickness of the enamel layers. Teeth gradually take up uranium from the groundwater after burial. In vivo they contain less than 1 mg/g U, but fossil ones up to 1,500 mg/g. This means that the dose rate increases with time and that the closed-system condition is not fulfilled. To allow for time dependence of the dose rate, distinct models of uranium uptake are assumed such as the early uptake (EU), the linear uptake (LU), and recent uptake (RU) models. The EU-model results in a lower ESR age compared to the LU model due to a higher dose rate on the average. Both model ages may considerably differ from each other, especially for high uranium contents so that samples low in uranium are preferable. Most published ESR-age data are based on assumed U-uptake models. In order to set constraints to the validity of the model, ESR dating is coupled with uranium series dating (Grün et al. 1988). The comparison the closed-system ages of 234U/238U and 230Th/234U with that of ESR enables to discriminate among the hypothetical models. Combined ESR/uranium series dating has become more and more the routine for tooth enamel dating.

The Early and Middle Paleolithic sites of Tabün, Kebara, Skhul, Qafzeh and Hayonim, Israel, with rich lithic and human bone inventories play a fundamental role with respect to the early human out-of-Africa dispersion through the Levantine corridor. In order to establish a firm chronology of this migration, ESR-dating has been repeatedly applied to mammalian tooth enamel from these sites, in addition to uranium series and luminescence. Worth mentioning in this connection are, in particular, two results. First, the combined ESR/uranium series tooth age of 387 ± 50 ka confirms a 340 ± 33 ka TL age on burnt flint for the Lower Paleolithic Yabrudian lithic industry at Tabün, whereby the U-uptake appears to be more recent than linear (Rink et al. 2004). Second, the ESR ages on teeth from Middle Paleolithic contexts support the early dating of the anatomically Moderns to 80-120 ka, with the amazing consequence that in the Levant the Moderns and Neanderthals coexisted for about 60 ka (Grün and Stringer 1991).

At the well-known South African site Swartkrans, with its wealth of remains of Australopithecus robustus, Curnoe et al. (2001) determined one of the oldest ESR ages so far reported. Coupled ESR/uranium series dating on two human and two bovid teeth yielded 1630 ± 160 ka with a possible maximum of 2110 ± 210 ka, indicating that ESR dating can provide reasonable results for samples of Late Pleistocene/Early Pleistocene age. Also the Atapuerca Gran Dolina site, Northern Spain, with the earliest humans in Europe, H. antecessor, was investigated by combined ESR/uranium series dating. From the bottom of Aurora stratum, where the very early human remains had been recovered, three ungulate teeth gave a mean of 731 ± 63 ka, which is in agreement with the paleomagnetic age estimate of >780 ka (Falgueres et al. 1999).

ESR also permits the dating of calcareous sinter deposits, which show complex ESR spectra partly attributed to organic radicals. Important Paleolithic cave sites have been dated, among them Caune de l'Arago at Tautavel, southern

France. ESR-dating of calcareous deposits under- and overlying the hominid-bearing bed allowed its bracketing between 242-313 ka as upper and 147 ka as lower age limits, respectively. This result is in good agreement with uranium series data of 315-220 ka (Hennig and Gran 1983). ESR-dating was also tried on travertine, e.g., for the profiles Weimar-Ehringsdorf and Bilzingsleben, central Germany, with important hominid fossils (Schwarcz et al. 1988). The data support Middle Pleistocene ages for this site but probably need some revision using the more advanced approach now available.

ESR may also be applied to quartz grains from clastic sediments provided the grains were exposed to light for about 6 months during sedimentary transportation so that the minimum signal level in Al-centers is reached. However, there are so far only a few examples of successful applications, one being the Early Pleistocene Monte Pogglio site, Italy, where more than 4,000 Paleolithic flint artifacts were found in sandy beach deposits. Detrital quartz extracted from different archeological levels provided a mean age of 1065 ± 165 ka, which is in agreement with paleomagnetic data (Falgueres 2003).

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